1. Field of the Invention
The present invention relates to an exhaust system for a motorcycle, and particularly, to an advantageous layout structure of an exhaust pipe manifold portion in a multi-cylinder engine.
2. Background of the Invention
It is well known to gather exhaust pipes of a multi-cylinder engine to a manifold portion forming an expansion chamber and allow exhaust gas to expand therein, thereby reducing an exhaust noise. Japanese Patent No. 3727641 discloses a structure, wherein the manifold portion is disposed below the engine. JP 07-046725Y discloses a structure, wherein the manifold portion is disposed between a rear portion of the engine and a rear wheel and below a rear swing arm. It has also been proposed to dispose a catalyst within the manifold portion to purify exhaust gas.
The manifold portion is relatively large in capacity. Therefore, if it is disposed below the engine, it becomes difficult to ensure a sufficient lowest road clearance. On the other hand, the manifold portion is heavy. Therefore, if it is disposed behind the engine, it becomes difficult to effect the mass concentration. The mass concentration is also strongly demanded together with lowering the center of gravity. The demand for mass concentration is also a basic and one of the most important demands in a body design of a motorcycle. The demand for lowering the center of gravity is also a basic and one of the most important, keen demands.
For attaining the mass concentration and the lowering of the center of gravity, it is preferable to dispose the manifold portion below the engine. To ensure a sufficient minimum road clearance, it is preferable to dispose the manifold portion behind the engine. These methods, however, are difficult to both ensure a sufficient minimum road clearance and attain the mass concentration and the lowering of the center of gravity to a satisfactory extent.